Using the manufacture of ethanol for its use in Scotch malt whisky as an example, crushed malted barley grain is used as a carbohydrate source and this is mixed with hot water to solubilise sugars present in the barley. For malt whisky, no grain other than malted barley is used. The lignocellulosic solid matter provided by such a process is separated to leave a sugar rich liquor known as “wort”. The wort can contain up to 20% protein in dry matter. The separated lignocellulosic solid matter (mainly malted grain known as “draft”) can be dried, or mixed with other additives for use as a versatile animal feed stock for cattle, sheep, pigs and horses.
In a typical distillation process, the wort is transferred to a large vessel(s) known as ‘washbacks’ and fermented by adding yeast. The yeast converts the sugar in the wort to ethanol, creating what is known as the ‘wash’ and the fermentation is allowed to proceed until the desired alcohol content, typically around 8%, is reached. At the end of the fermentation process, the fermented wash is charged to a first distillation vessel (the wash still). In traditional Scotch malt whisky manufacture, the distillation vessels or stills are made of copper (Cu) metal whereas for grain whisky, sacrificial copper components may be used. Copper is commonly used due to its ability to be easily shaped into intricate designs and its rate of heat transfer. The stills are heated to drive off the ethanol and the ethanol is then condensed to a liquid. This distillate is then charged to a second distillation vessel (the spirit still). In the spirit still, the ethanol is re-distilled to increase its purity before it is cooled and recovered as a liquid for maturation in oak casks. By law, it cannot be Scotch whisky until it has matured in oak casks in Scotland for at least three years.
Once the first distillation is complete, the liquid residue that remains in the wash still after distillation is known as “pot ale” or less commonly “burnt ale”. Pot ale is a low pH liquid that contains dead yeast cells, yeast residues, carbohydrates, protein material from the yeast, microbes (Lactobacillus species) and the grain residues and extracts.
Whilst the pot ale by-product generated may be rich in nutrients it typically has a high chemical or biological oxygen demand (COD or BOD) making the pot ale by-product difficult to process and dispose.
Further, during brewing and distilling, a small amount of copper metal of the still typically transfers to the pot ale through abrasion or dissolution where it remains in solution or bound to suitable ligands. Whilst copper is an essential trace element for life, it is well understood that copper (and other materials) are bio-accumulative in the livers of certain mammals (e.g. sheep and goats) and that this bio-accumulation can reach acute/toxic levels.
With a low solids content, pot ale may be concentrated using evaporation or other means for use as an animal feed additive known as “pot ale syrup”. Evaporation is an energy intensive method which suffers from high cost. Additionally evaporation can suffer from high greenhouse gas emissions and the high temperatures used may have a deleterious effect on protein quality. Evaporation concentrates all the components in pot ale, including non desirable compounds such as phytate and copper. Typically, the pot ale syrup is mixed with the recovered draff and dried to make animal feed known as distillers' dark grains. However, in view of the presence of copper the use of pot ale syrup and distillers' dark grains as feeds is restricted to cattle, horses and pigs. Examples of the use of pot ales as a feed additive and pot ale processing methods are described in GB2094084 (Gilmour et al), EP1927291 (Atherton et al) and IE980676 (Court et al).
Pot ale by-products generated from grain fermentation may contain at least 20% protein as dry matter. It has been realised that this protein has a commercial value as a feed additive and efforts to recover this protein by precipitation or flocculation at a high pH have been employed. In the process described in GB2094084, the pot ale is treated with a caustic material (NaOH, KOH or Ammonium Hydroxide) to raise the pH to greater than neutral pH causing flocculation of the protein. The protein is then allowed to settle before mechanical recovery. This requires a large volume of caustic material, can be expensive and furthermore, not all protein is recovered using this process, since only proteins with iso-electric pH lower than neutral pH will precipitate, whereas those above neutral pH will still remain in solution. A protein recovery of 50-60% is typical.
In GB1347933 (NESTLE) a multistep process is discussed wherein an alkaline protein solution is filtered through a macro-reticular ion exchange resin before removing the alkali (Ammonia, NH3) through heat treatment to give purified protein. A disadvantage of this process is that it is limited to alkaline pH starting material.
It would be advantageous if by-products arising from distillation processes could be tailored for specific animal and fish feed applications.
Whilst, distillery by-products have been used in aquaculture (Randall and Drew, 2010 [Randall, K. M., Drew, M. D., 2010. Fractionation of wheat distiller's dried grains and solubles using sieving increases digestible nutrient content in rainbow trout. Anim. Feed Sci. Technol. 159, 138-142];
Shurson 2012 [Shurson, J., 2012. Maize dried distillers grains with solubles (DDGS)—a new alternative ingredient in aquafeeds. World Aquaculture. September 2012. pp. 54-58]), their use is restricted by the high fibre content (at the expense of protein) and also the presence of phytate with supplementation with essential amino acids such as lysine and methionine or phytase required to achieve higher incorporation levels in the total fish feed (Reveco et al., 2012 [Reveco, F. E., Collins, S. A., Randall, K. M., Drew, M. D., 2012. Aqueous fractionation improves the nutritional value of wheat distillers dried grains for rainbow trout (Oncorhynchus mykiss). Aquacult. Nutr. 18, 202-210.]; Stone et al., 2005 [Stone, D. A. J., Hardy, R. W., Barrows, F. T., Cheng, Z. J., 2005. Effects of extrusion on nutritional value of diets containing corn gluten meal and corn distiller's dried grain for rainbow trout, Oncorhynchus mykiss. Journal of Applied Aquaculture 17:1-20]; Cheng et al., 2003 [Cheng, Z. J., Hardy, R. W., Blair, M., 2003. Effects of supplementing methionine hydroxyl analogue in soybean meal and distiller's dried grain-based diets on the performance and nutrient retention of rainbow trout [Oncorhynchus mykiss (Walbaum)]. Aquaculture Research 34:1303-1310.]; Cheng and Hardy 2004a [Cheng, Z. J., Hardy, R. W., 2004a. Effects of microbial phytase supplementation in corn distiller's dried grains with solubles on nutrient digestibility and growth performance of rainbow trout (Oncorhynchus mykiss). Journal of Applied Aquaculture 15:83-100.]; Cheng and Hardy 2004b [Cheng, Z. J., Hardy, R. W., 2004b. Nutritional value of diets containing distiller's dried grain with solubles for rainbow trout (Oncorhynchus mykiss). Journal of Applied Aquaculture 15:101-113.]).